Infrared absorption gauges are well known and are used for example for measuring constituents of samples (e.g. the moisture content of paper or tobacco, or the fat, protein and water contents of foodstuffs), the amounts of substances absorbed or adsorbed on a substrate, the thickness of coatings or films on a substrate or the degree of cure of resins in a printed circuit board. In this specification, the term “parameter” is used to denote the property (composition, coating thickness etc.) of the sample being measured.
Infrared absorption gauges conventionally operate by projecting infrared radiation at two or more wavelengths onto a sample or a substrate and measuring the intensity of the radiation reflected, transmitted or scattered by the sample. Signals proportional to the measured intensity are processed to provide a value of the parameter being measured. At least one of the two or more wavelengths projected by the gauge is chosen to be absorbed by the parameter of interest while at least one other wavelength is chosen to be substantially unaffected by the parameter of interest. For example, when measuring the amount of water in a sample, one of the wavelengths (the “measuring wavelength”) can be chosen at an absorption wavelength of water (either 1.45 micrometer or 1.94 micrometer) and the other wavelength (known as the “reference wavelength”) is chosen to be one that is not significantly absorbed by water.
Generally, gauges include an infrared radiation source having a broad emission spectrum and a detector for receiving radiation reflected, scattered or transmitted by the sample; filters are placed between the source and the sample to expose the sample only to the desired measuring and reference wavelengths; in this case, the sample is successively exposed to radiation at the selective wavelengths, e.g. by placing appropriate filters on a rotating wheel in front of the radiation source. Alternatively, the filter wheel can be placed between the sample and the detector and each filter is successively interposed between the sample and the detector. Naturally, if the source can produce radiation of the desired wavelength without the use of filters, then such filters can be dispensed with.
The detector measures the intensity of light after interaction with the sample and produces a signal according to the intensity of the radiation incident upon it. In the most simple case, by calculating the ratio between the signal from the detector when receiving light at the measuring wavelength to that when receiving light at the reference wavelength, a signal can be obtained that provides a measure of the parameter concerned, for example the amount of moisture in a sample. Often, several measuring wavelengths and/or several reference wavelengths are used and the signals of the measuring wavelengths and of the reference wavelengths are used to calculate the parameter concerned.
The detectors which are normally used in such measuring gauges are conventionally lead sulphide (PbS) detectors, because they display better detectivity and wavelength response than most other detectors which might be employed in such applications. However, PbS detectors have a number of limitations, including particularly the following:                (a) Temperature sensitivity: the resistance of a typical detector cell falls by 25% for every 10° C. rise in temperature.        (b) Non-linearity: the response of the detector to incident radiation is not linear over the whole operational range of the detector.        (c) Response time: the response time of the detector usually limits the rate at which different wavelengths can be detected, that is the rate at which successive filters can be employed. Faster filter data rates tend to result in the signal from the wavelength obtained from one filter lagging so much that it bleeds into that from the wavelength obtained from the next filter, thereby causing “cross-talk”.        (d) Noise: at low frequencies of operation of the detector a type of noise known as 1/f noise predominates. If a relatively low filter data rate is chosen to avoid cross-talk, then such noise becomes a problem.        
It is apparent from the, above that the detectors currently used in measuring gauges suffer from a number of drawbacks, not the least of which is their response time.
The present invention seeks to address these problems and to improve the performance of the detectors employed in electromagnetic detection apparatus, such as infrared measuring gauges.